Technical Field
The present invention relates to an image processing technique and in particular to an image processing technique involving screen processing.
Related Art
There are techniques for representing halftones by performing screen processing (dithering).
In the case of representing characters using such a technique, “jagged edges” (jaggies) may appear in character outlines if the directions in which the character outlines extend and the direction in which a screen is oriented (the direction corresponding to a predetermined screen angle) are close to parallel to each other.
As a technique for reducing such jagged edges, there is, for example, a technique disclosed in JP 2008-294609A.
JP 2008-294609A discloses that the font of a character is analyzed, and if a difference between a line segment angle of the font and the screen angle is less than or equal to a threshold value, it is determined that jaggies will appear, and every screen for each color is rotated by 90 degrees to generate an output image.
With the above-described technique disclosed in JP 2008-294609A, screen processing using a new screen angle obtained by rotating the original screen angle by 90 degrees is performed on each page. More specifically, an angle obtained by 90-degree rotation of a character angle that has the highest frequency of appearance on each page is determined as a new screen angle, and screen processing using the new screen angle is uniformly performed on all characters on that page.
Accordingly, the above technique does have the effect of reducing jaggies in line segments having a predetermined angle on each page (specifically, a character angle with the highest frequency of appearance on that page).
However, because there are line segments of various angles on each page, jaggies may not be reduced in other line segments whose angles are different from the above predetermined angle.